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Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Cover of C. elegans II

C. elegans II. 2nd edition.

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Section IVAcquisition and Mechanism of Spermatozoan Motility

Nematode spermatozoa differ substantially from vertebrate and many other invertebrate spermatozoa by lacking both a flagellum and acrosome (Fig. 2) (for review, see Foor 1970, 1983; Heath 1992; Roberts and Stewart 1995). Spermatozoan motility in C. elegans and Ascaris has been reviewed previously (see, e.g., Ward et al. 1982; Roberts et al. 1989; Roberts and Stewart 1995). The amoeboid motility exhibited by C. elegans and other nematode spermatozoa is unusual in that a single pseudopod with a position that is fixed relative to the cell body is employed (Fig. 2). The availability of both in vitro activators and a suitable medium has permitted detailed in vitro analyses of spermatozoan motility (for review, see Ward et al. 1982). Translocating cells are attached to the substrate by knobs on their pseudopodial surface, whereas the cell body is unattached (Nelson et al. 1982; Roberts and Streitmatter 1984). The pseudopod promotes forward movement by directed bulk membrane flow, which includes glycoproteins and lipids. These movements have been studied by nonspecific probes, such as latex beads and fluorescent lipid probes (Roberts and Ward 1982a,b), as well as by monoclonal antibodies to membrane proteins (Pavalko and Roberts 1987, 1989). The results all indicate that membrane components are inserted at the tip of pseudopodial projections and move back toward the junction of the cell body and pseudopod. The rate of bulk membrane flow closely approximates the rate of forward movement (~20–30 μm per minute). It is not known if surface components are internalized and recycled.

A. Major Sperm Protein and the Cytoskeleton

Nematode spermatozoa are unusual among crawling cells in that they lack appreciable amounts of most conventional cytoskeletal proteins (for review, see Roberts et al. 1989). In C. elegans, less than 0.02% of sperm protein is actin, no microfilaments can be detected, and there is no detectable myosin in bulk preparations of spermatids or spermatozoa. The trace amount of actin probably resides mostly within the few spermatocytes that always contaminate spermatid preparations, although small dots of staining are also detected in pseudopods (Nelson et al. 1982). Spermatozoan motility is unaffected by cytochalasin B, D, or E under conditions where spermatocytes (which do contain actin) do not properly divide. C. elegans spermatids and spermatozoa also do not have microtubules except those that form the pair of centrioles (Wolf et al. 1978; Ward et al. 1981; Ward 1986). Not surprisingly, the tubulin-depolymerizing drugs colchicine and oncobendazole are without effect on spermatozoan motility. Yet, as mentioned above, the vigorous surface movements and remodeling of the C. elegans pseudopod that occur as the cell moves forward suggest cytoskeleton-mediated membrane flow.

Electron microscopic examination of the pseudopod reveals a dense, granular cytoplasm devoid of organelles, which are confined to the cell body (Fig. 2). Under certain conditions of preparation for ultrastructural analyses, it is possible to observe approximately 2-nm filaments that form a loose meshwork within the granular cytoplasm (Roberts 1983). One of the major components of the pseudopod is MSP (Ward and Klass 1982; Roberts et al. 1986), a protein that comprises about 10–15% of total sperm protein (Klass and Hirsh 1981). The multigene family that encodes the C. elegans MSPs has been extensively analyzed and is highly conserved (Burke and Ward 1983; Klass et al. 1984, 1988; Ward et al. 1988). Unfortunately, the small size and dense distribution of fine filaments within the pseudopod have made it difficult to use immunogold localization to determine the identity of the 2-nm filaments in vivo. However, polyclonal (Klass and Hirsh 1981) and monoclonal (Roberts et al. 1986) antibodies to C. elegans MSP cross-react with similar proteins in Ascaris (for review, see Roberts et al. 1989). In Ascaris, anti-MSP antibodies decorate filament bundles in the pseudopod that extend into substrate contact areas and play a dynamic part in cell motility (Sepsenwol et al. 1989).

Purified Ascaris MSP will assemble into 11-nm filaments in vitro (King et al. 1992, 1994a; Stewart et al. 1994). Filament assembly is pH-sensitive (Roberts and King 1991), and a pH gradient in the Ascaris pseudopod might modulate MSP assembly and filament/membrane interactions in vivo (King et al. 1994b). The sequence of MSPs from Ascaris (Bennett and Ward 1986; King et al. 1992) is 82% identical to the deduced consensus derived from C. elegans MSP gene sequences (Bennett and Ward 1986; Ward et al. 1988). Preliminary data suggest that C. elegans MSP proteins also assemble into filaments in vitro (M. Smith et al., pers. comm.). Ascaris MSP filament bundles move retrograde (from the pseudopodial tip toward the cell body) with a velocity that is similar to the retrograde movement of psudopodial surface components (Roberts and Ward 1982a,b; Royal et al. 1995). Membrane components isolated from the leading edge of the pseudopod appear to facilitate MSP assembly into filament bundles in vitro (Italiano et al. 1996). These bundles are then capable of moving vesicles and, as is true in vivo for both C. elegans (Ward et al. 1983) and Ascaris (Roberts and King 1991), motility is dependent on ATP production. These observations suggest that an ATPase(s) and/or kinase(s) plays an important part, but no candidate proteins have yet been identified from either species.

MSP association with membranes appears to be analogous to actin association with membranes in other amoeboid cells (for review, see Condeelis 1993), with some important differences. Unlike actin, the ATP requirement for MSP filament function is indirect, since the protein does not contain a nucleotide-binding consensus motif (King et al. 1992) and does not seem to bind ATP in solution (J.E. Italiano and P. Fajer, unpubl.). The sperm MSP cytoskeleton also has no known analog of myosin, which can cross-link actin filaments and provide contractile activity in many types of cells (for review, see Ruppel and Spudich 1995). However, myosin might have a secondary role in membrane expansion during amoeboid movement (for review, see Zigmond 1993), and actual protrusive activity could be based on specific actin filament–membrane interactions, as is the case for MSP-membrane interactions (Italiano et al. 1996). How membrane interactions lead to actin-based protrusion in amoeboid cells is unclear. For instance, ponticulin is the membrane protein that accounts for nearly all actin-plasma membrane interactions in Dictyostelium, but mutants lacking this protein can still crawl (Shariff and Luna 1990; Chia et al. 1993; Hitt et al. 1994).

B. Mutants Defective in Motility

In most spe mutants, sperm are defective in motility because they fail to make pseudopods. However, a few mutants make sperm that exhibit defective motility despite extending pseudopods. Such motility-defective sperm have cytological defects in either MOs (caused by fer-1 or spe-10 mutation) or perinuclear material (caused by fer-2 , fer-3 , or fer-4 mutation). However, the connection between these defects and motility, if any, is unclear (Ward et al. 1981; Shakes and Ward 1989b). Abnormally short pseudopods are present on fer-1 and spe-10 spermatozoa. These pseudopods do not move properly in either mutant, and neither fer-1 nor spe-10 spermatozoa crawl across the substrate in vitro. fer-1 spermatozoa appear to be defective in the directed membrane flow characteristic of wild-type spermatozoan motility (Roberts and Ward 1982a,b). Latex beads bound to the pseudopodial surface of fer-1 sperm frequently move randomly and can move across the pseudopod/cell body junction; beads on pseudopods of wild-type cells always move from tip to pseudopod/cell body junction and never cross this junction. Short pseudopods per se do not preclude active motility. spe-17 spermatozoa are only about 66% wild type in size and have abnormally short pseudopods, yet they are able to crawl in vitro and in vivo (Shakes and Ward 1989b; L'Hernault et al. 1993). fer-2 spermatozoa either lack pseudopods or have pseudopods of variable morphology, for example, short or helically twisted (Ward et al. 1981). fer-3 and fer-4 mutant sperm make pseudopods that frequently appear normal ( fer-3 ) or have abnormally few pseudopodial projections ( fer-4 ) (Ward et al. 1981). Lectin-binding experiments with fer-2 , fer-3 , and fer-4 sperm reveal that membrane glycoproteins move much more slowly across the pseudopodial surface of any of the three mutants when compared to wild-type cells (Argon 1980; Nelson et al. 1982; Roberts and Ward 1982b).

Copyright © 1997, Cold Spring Harbor Laboratory Press.
Bookshelf ID: NBK20042
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